In many laser applications precise control of beam output is desired. One such application for such lasers is the light source for integrated circuit lithography. Currently the KrF excimer laser is the choice light source for state of the art integrated circuit lithography devices. Specifications for the light source are becoming tighter as efforts are made to increase production and produce finer integrated circuit patterns.
Typical specifications for a 248 nm KrF laser call for bandwidths of about 0.6 pm full width half maximum, wavelength stability within 0.1 pm of the specified wavelength and energy dose stability of about .+-.0.5 percent. In addition, control of beam cross section intensity values are important.
FIG. 1 shows some of the features of a prior art KrF excimer laser system used for IC lithography. The system includes a laser frame structure 5 within which is mounted a laser chamber 3 containing two elongated electrodes (not shown) between which is a gain medium, a line narrowing module (referred to as a "line narrowing package" or LNP) 7 shown disproportionately large and an output coupler 4. The LNP portion of FIG. 1 represents a top view of the LNP. The beam cross section is generally rectangular, typically about 3.5 mm wide and about 15 mm high. In prior art devices each of the line narrowing module 7 and the output coupler module 4 comprise frames which are fixedly mounted to laser frame structure 5. Optical components within the frames of the output coupler module and the line narrowing module are adjusted manually to define the laser resonance cavity. The chamber is adjustably mounted within the laser frame so that it can be finely positioned manually within the defined resonance cavity from time to time in the direction of the beam width as shown by arrows 3A on FIG. 1. These adjustments permit a laser technician to align the resonance cavity with the gain medium in order to achieve optimum beam output parameters. In this prior art for example, a three prism beam expander 18 is comprised of prisms 8, 10 and 12 mounted on prism plate 13. In the prior art device, prism plate 13 can be manually adjusted in the direction of arrows 13A as an alignment technique. The prior art device also includes a manual adjustment of the curvature of the surface of grating 16 into an increasingly or decreasingly concave shape by expanding or contracting bending mechanism 20 to place larger or smaller compressive forces on legs 17A and 17B. The adjustment is done primarily to control bandwidth of the output beam. Another prior art technique for forcing a concave shape on the grating surface is described in U.S. Pat. No. 5,095,492.
Typical prior art lithography excimer lasers now in use incorporate two automatic feedback controls to regulate pulse energy and nominal wavelength. Pulse energy is controlled in a feedback system by measuring the output pulse energy with a beam output monitor 22 as shown in FIG. 1 and then using these measurements with a computer controller 24 to control the high voltage applied between the electrodes in order to regulate pulse energy within desired limits. The beam output monitor 22 (also called a wavemeter) also measures the nominal wavelength and bandwidth of the pulsed output beam. Computer controller 24 adjusts the pivot position of tuning mirror 14 using stepper motor 15 in order to control the nominal wavelength of the beam to within desired limits.
What is needed are improvements which will provide easier, faster and more precise control of laser beam output parameters.